Generally speaking, the best (free) on-line source is Douglas Harper's Online Etymology Dictionary (subscribers can naturally refer to the Oxford English Dictionary), but here he doesn't provide much more than what you have written above :

snorkel1944, "airshaft for submarines," from Ger. Schnorchel, from Ger. navy slang Schnorchel "nose, snout," related to schnarchen "to snore" (see snore). So called from its resemblance to a nose and its noise when in use. The Anglicized spelling first recorded 1949. The meaning "curved tube used by a swimmer to breathe under water" is first recorded in 1953.

... The meaning "curved tube used by a swimmer to breathe under water" is first recorded in 1953.

But not too far under water. Remember, pressure increases very quickly - i e, beyond the capacity of human lungs and diaphragm to compensate for - with increasing depth in water. What the snorkel does is allow a swimmer at the surface to breathe with his or her face continually in the water, which obviously facilitates obervation of what is going on down there. If one wants to go to below the surface, one either has to hold one's breath or use scuba equipment....

Henri, who used to do a fair amount of diving on Bronze Age wrecks and stuff like that...